Residue mutations in the sweetness loops for the sweet-tasting protein brazzein

To identify critical residues, important for sweetness, of the sweet protein brazzein, 11 mutants of the residues in three loops of brazzein were constructed by site-directed mutagenesis. We found that mutations of Glu41 to Ala, Lys, or Arg at position 41 in loop 40–43 made the molecules significantly sweeter than brazzein, while mutations at two distant residues (changing Arg43 to Lys or Glu) decreased sweetness. A similar pattern occurred at loop 30–33, where mutation of the His31 to Arg significantly increased sweetness, while mutations at positions 30 or 33 in the immediate vicinity of this region significantly decreased sweetness. In addition, a Gln17 residue in the loop 9–19 was necessary for structural integrity. From these results, we suggest that the loops containing His31 and Glu41 are critical regions of the molecule for eliciting sweetness, and the charge and/or structure of the side chain of these residues play an important role in the multi-point interactions between brazzein and the sweet-taste receptor.